In typical gas turbine power generating systems, a governor or speed control is normally used to alter the fuel flow to the gas turbine to maintain turbine speed at preselected value under varying ambient and load conditions. By comparing actual turbine speed to a reference or setpoint value, sometimes called a speed changer under control of an operator, a speed error comprising the difference between the actual and reference values is determined. The change in fuel flow to the gas turbine is a function of this error. The output of the gas turbine can therefore be controlled by the operator by changing the speed error, i.e., by raising and/or lowering the reference value or setpoint.
This type of speed control is referred to as droop control or a droop governor, and the fuel flow to the gas turbine is a function of the error signal. For example, if a zero error signal exists, very little fuel is supplied the turbine inasmuch as it requires a certain minimum amount of fuel at no-load to operate the turbine. Thus, a zero error signal corresponds substantially to operation at no-load fuel. To obtain positive output, additional fuel must be supplied through an error signal between actual and reference speeds. If a positive error is set into the system by adjusting the setpoint, additional fuel is supplied the turbine in proportion to a gain constant. For example, to go from full speed no-load fuel to full speed full-load fuel, an error of 5% may be set into the governor, assuming a 20:1 gain constant, to obtain 100% fuel or full speed full-load fuel. The speed error between no-load to full-load is called droop. In the example, the error between the reference speed and the actual speed must be equal to the droop, i.e., 5%, to obtain 100% fuel, and 0% to obtain full speed no-load fuel in the turbine.
In systems of this type, the controlled fuel flow output to the turbine is measured as part of the feedback to the speed control. The energy content of the fuel, however, varies for example, in accordance with the vagaries of fuel temperature, heating value and frequently steam or water injection flow rates for low NO.sub.x operations. More specifically, fuel heating value variations are commonly encountered where fuel gas is produced as a petrochemical process by-product with variations in gas composition, where fuel gas is supplied from several different sources, or where fuel is supplied from a single field whose composition changes with age or acquisition depth. Fuel conversion efficiency to power output also changes as a result of turbine aging or, in a more pronounced fashion, as a result of changing combustion efficiency during mode changes employing a dry low NO.sub.x combustion system. As a consequence, measuring the fuel flow rate and providing a feedback signal to the speed governor responsive to the fuel flow rate does not accurately reflect the needed fuel to obtain the particular setting. However, because a gas turbine makes an excellent calorimeter, the generator output will reflect any change in the energy input at steady fuel flow command. Utilization of output power feedback in place of fuel flow feedback is called constant settable droop. Thus, a settable fractional feedback of generator output power is employed in place of the traditional fuel flow feedback using a constant settable droop control system. If a signal proportional to a percentage of the generator output is fed back to the governor in place of the fuel flow feedback, the result is a corresponding percent proportional droop system that will maintain this proportionality independently of fuel or fuel injection conditions. Constant settable droop is therefore a feature in which measured power output of the generator is substituted for fuel command feedback to cause a proportional or droop response of the speed governor to an error between speed setpoint and actual speed. It is employed where there is a deviation in the proportionality between gas turbine fuel command (fuel volume) and output power of the generator. Constant settable droop governing systems have been used for many years to compensate for these conditions on simple cycle gas turbines.
In modern-day energy production, combined-cycle systems are often employed. A typical combined-cycle system employs one or more gas turbines in conjunction with a steam turbine for driving a generator. One such typical combined-cycle system employs a steam turbine, a gas turbine and a generator on a single shaft. A constant settable droop control system for the gas turbine, however, cannot be employed directly in that the generator is driven by both the gas turbine and steam turbine which have widely varying dynamic responses to energy release within the gas turbine. Consequently, the generator output does not directly represent the gas turbine output as in simple-cycle gas turbine systems. This is especially true during simultaneous dynamic processes of loading and unloading the gas turbine through dry low NO.sub.x mode changing points. The slower response of the steam cycles alters the relationship. As a consequence, constant settable droop governing systems, as traditionally used in simple-cycle gas turbines have not heretofore been directly applicable to combined-cycle power generating systems.